1. Field of The Invention
This invention realtes generally to thermally-enhanced oil recovery methods, and more particularly to a method and apparatus for thermally-enhanced oil recovery of deep well reservoirs utilizing electric downhole steam generators to provide supplemental heat to a flow of high pressure hot water to generate high quality steam.
2. Background Information
Generally lowering crude oil prices make it economically difficult to justify development of new oil fields. Most of the new oil field developments are likely to be in remote or off-shore areas, with high exploration and field operating costs. Thermally-enhanced oil recovery methods that are applied to already discovered domestic heavy oil fields have an in-place infrastructure and a near-by market. Also, when crude oil prices are stabilized, the efficiency of the thermal recovery process for heavy oil production keeps the latter competitive at lower prices.
The economics of thermally-enhanced oil recovery can also be significantly improved when cogeneration of electricity is considered. It is believed that this will be particularly advantageous in deep reservoir regions such as the Texas and Mississippi areas as well as in California with the established infrastructure, the need to continue oil production and ready markets for both crude oil and electric power.
California has a large number of suitable deep reservoirs, but it also has many shallow reservoirs, which have not yet been fully exploited. In Texas and Mississippi, however, 90% of the suitable reservoirs are at dephths below 2500 feet. In order for these states to keep their rate of oil production, they must depend increasingly upon enhanced oil recovery. Since thermally-enhanced oil recovery or steamflooding is one of the most efficient, advanced, and economical of enhanced oil recovery methods, this process is one that will be used more often.
Because of the heat loss in conventional bare steam injection pipes, it is difficult to supply steam efficiently to reservoirs deeper than 2500 feet. The steam pipe is relatively large when compared to the typical 7-inch well bore dimension. The steam pipes must be installed in sections and therefore, space must be allowed for the screw joints between sections. The pipes must also be large enough to supply the steam with a relatively low pressure drop. For example, at reservoir depth of 2500 feet, the reservoir pressure is over 1000 psia. Since the steam is a low density fluid, there is little help from its hydrostatic head (40 psi). Insulated piping (double-walled) is often used, but this increases the space problem.
Other methods of transporting the heat downhole have been suggested which have the combustion occur at the reservoir face. Such downhole burners have been tried experimentally with limited success. Other systems which transport fuel, feed water, and oxidizer (usually air) downhole where combustion occurs have been suggested also. One system uses the hot gases to boil water with a heat exchanger so that the combustion can occur at nearly atmospheric pressure. This system cannot exhaust the cooled exhaust gas into the reservoir because its pressure is too low. Therefore, it must be transported back up the well bore to the surface. Any gaseous pollution products must then be handled at each injection well.
Another system carries out the combustion at a pressure greater than the reservoir pressure which permits the combustion products to be discharged into the reservoir. This system requires the compression of both the fuel and oxidizer as well as solving the technical problem of carrying out the combustion at very high pressures. The control of the combustion and water boiling processes in restricted dimensions at a distance up to a mile in the earth poses severe technical problems. While these technical problems may be solved, there is concern about the ability to operate these devices practically in an oil field environment.
3. Brief Description of the Prior Art
There are several patents which disclose various systems of thermally-enhanced oil recovery utilizing electrical steam generators for heating injection fluids or production fluids.
Stegmeier, U.S. Pat. No. 2,932,352 discloses multiple heating elements circumferentially placed about an axially extending conduit, the elements being divided into groups of three with each group being supplied with a single phase of three-phase alternating current and the elements of each group being electrically connected at the bottom. The heater of the Stegemeier patent is used to heat fluids residing in a reservoir.
Curson, U.S. Pat. No. 2,754,912 discloses another system having multiple heating elements circumferentially placed about an axially extending conduit, the elements being divided into groups of three with each group being supplied with a single phase of three-phase alternating current and the elements of each group being electrically connected at the bottom. The heater of Curson is used to heat fluids being produced through an oil stem.
Schlinger, U.S. Pat. No. 4,007,786 discloses a secondary recovery process using steam as a stimulation fluid, the steam being generated by sensible heat recovered from a gas turbine which optionally may be used to drive an electric generator for providing electrical energy.
Tubin et al, U.S. Pat. No. 4,127,169 discloses a secondary recovery process using an electrically-powered downhole steam generator providing thermal stimulation of deep reservoirs. The system does not use surface steam lines or a boiler. Cold water is pumped down the tubing string to be converted to steam.
Gill, U.S. Pat. No. 3,614,986 discloses a recovery process including flowing electrical current through an injection turbine used to convey heated fluids to a mineral bearing formation and thereby producing sufficient heat in the turbine to prevent heat loss from the injection fluids while they move through the turbine.
The present method for exploiting deep-well reservoirs utilizing electric downhole steam generators is distinguished over the prior art by its provision of a thermally efficient system for adding heat to high pressure hot water. The downhole steam generators are powered by electricity from above-ground turbine-driven electric generators fueled by any clean fuel, possibly from the production field itself. The downhole steam generators include multiple heating elements circumferentially disposed around an axial, insulated, small-diameter injection tube, the heating elements being divided into three groups with each group being supplied with a separate phase in a three-phase "Y" alternating current electrical system. The injection tube is closed at the bottom and contains radial orifices so that the injection fluid (pressurized hot water) flows between the heating elements to generate high quality steam. This steam then exits the heater assembly and flows into the oil reservoir that is being thermally stimulated. Heat recovered from the gas turbine exhaust is used to provide pressurized hot injection water, and, when desired, electrical power may be sold to an electric utility to provide an immediate cash flow and improved economics.